Known methods of forming a metal film include so-called dry processes such as vapor deposition, sputtering, and ion plating, so-called wet processes such as electroplating and electroless plating, and the like, but the dry processes demand large facility cost, while it was difficult to prepare a metal film having a thickness of several dozen nm by such a wet process.
Thus reported is a method of forming a metal film by generating a cation-exchange group by modification of a polyimide resin, binding a metal ion to the cation-exchange group, and reducing the metal (Patent Document 1). However, such a method forms a film by modification of polyimide and does not allow use of other substrates. A method of obtaining a hardened film after application of polyimide varnish on a substrate may be possible, but it demands baking at high temperature (e.g., at 200° C. or higher) for hardening and does not allow use of a substrate lower in heat resistance because of deformation (less general-purpose properties). In addition, the polyimide resins are rather expensive. Common polyimide resins (e.g., Kapton and Upilex) are also less transparent to light in the UV light range, and transparent polyimide resins are resistant to modification. When an electric circuit is formed, for example, in a metal pattern formed through a mask by the methods above, the low transmittance to UV light, which leads to deterioration in resolution, is not suitable for fine wiring. Further, use of a heated high-concentration alkaline solution, for example at dozens of temperature, causes a problem of safety.    Patent Document 1: Japanese Unexamined Patent Publication No. 2001-73159